Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystal framework structure of [SiO2], [AlO2], and [PO2] corner-sharing tetrahedral units. The [PO2] tetrahedral units are provided by a variety of compositions including phosphoric acid, organic phosphates such as triethyl phosphate, and aluminophosphates. The [AlO2] tetrahedral units are provided by a variety of compositions including aluminum alkoxides such as aluminum isopropoxide, aluminum phosphates, aluminum hydroxide, sodium aluminate, and pseudoboehmite. The [SiO2] tetrahedral units are provided by a variety of compositions including silica sols and silicon alkoxides, such as, tetraethylorthosilicate and fumed silica.
SAPO-34 and SAPO-18 have been reported as suitable catalysts for light olefin production from methanol. SAPO-34 belongs to the family of molecular sieves having the structure type of the zeolitic mineral chabazite (CHA). The preparation and characterisation of SAPO-34 has been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis,” Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry,” Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters,” Vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions,” Vol. 90 (15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions,” Vol. 86(2), pp. 425-429 (1990), all of which are fully incorporated herein by reference.
U.S. Pat. No. 6,334,994, fully incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed-phase composition. In particular, RUW-19 is reported as having peaks characteristic of both CHA and AEI framework-type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2, and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown phases of AEI and CHA framework-type molecular sieves with AEI/CHA ratios of about 60/40, 65/35, and 70/30, respectively. Throughout this description, the XRD reflection values are referred to as (2θ), which is synonymous to the expression “degrees 2θ.”
U.S. Pat. No. 6,812,372, fully incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein said intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve.
U.S. patent application Ser. Nos. 10/092,792 and 10/995,870, disclose a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the silicoaluminophosphate molecular sieve. It also relates to methods for its preparation and to its use in the catalytic conversion of methanol to olefins.
U.S. Pat. No. 6,773,688 and U.S. patent application Ser. No. 10/882,741 disclose a process for manufacturing a SAPO-34 silicoaluminophosphate molecular sieve, the process comprising the steps of: (a) providing sources of aluminium, of phosphorus and of silicon, wherein the source of silicon is in solution with a water-miscible organic base; (b) forming a synthesis mixture from the sources; and (c) treating the synthesis mixture for a period of time and at a temperature sufficient to form the silicoaluminophosphate molecular sieve.
U.S. patent application Ser. No. 10/985,496 discloses a method of synthesizing a silicoaluminophosphate molecular sieve, by preparing a synthesis mixture by combining a source of phosphorus and at least one organic directing agent; and then cooling the combination of the phosphorus source and organic directing agent to a temperature of less than or equal to 50° C., prior to introducing a source of aluminum into the combination. After addition of a source of silicon, the synthesis mixture is heated to a crystallization temperature of between about 100° C. and about 300° C. and the molecular sieve is recovered.
U.S. patent application Ser. No. 11/048,072 discloses a method of synthesizing a silicoaluminophosphate molecular sieve comprising a CHA framework-type material, an AEI framework-type material, or a material comprising at least one intergrown phase of an AEI framework type and a CHA framework type, the amount of alkali metal present in said synthesis mixture is controlled so as to reduce the crystal size of the molecular sieve and/or to increase the AEI character of the intergrown phase.
U.S. patent application Ser. No. 10/425,587 discloses methods and compositions of synthesis mixtures for the synthesis of aluminophosphates and silicoaluminophosphate molecular sieves, which enable the control and adjustment of the crystal particle size of aluminophosphates and silico-aluminophosphate molecular sieves. The synthesis mixture compositions used have two or more organic templates present at a molar ratio of total template to aluminum of ≦1.25; such a synthesis mixture is susceptible to control of product particle size through variation in the amount of seeds used in the synthesis.
U.S. patent application Ser. No. 11/072,830 discloses a silicoaluminophosphate molecular sieve that comprises first and second intergrown phases of a CHA framework type and an AEI framework type, wherein said first intergrown phase has an AEI/CHA ratio of from about 5/95 to about 40/60 as determined by DIFFaX analysis, the second intergrown phase has an AEI/CHA ratio of about 30/70 to about 55/45 as determined by DIFFaX analysis and said molecular sieve has a silica to alumina molar ratio (Si/Al2) from about 0.13 to about 0.24.
U.S. Pat. No. 6,767,858 discloses a method of synthesizing aluminophosphate and silicoaluminophosphate molecular sieves and, in particular, the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves using N-methylethanolamine as template with or without a source of fluoride. The use of N-methylethanolamine as template results in good quality AlPO4 of CHA framework type and SAPO molecular sieves of CHA framework type with low levels of silicon in high yield.
The present invention is related to a process for making silicoaluminophosphate molecular sieve that comprises at least one intergrown phase of molecular sieves having AEI and CHA framework types. The silicoaluminophosphate molecular sieve is made from a gel mixture having a molar composition of:(n)SiO2/Al2O3/(m)P2O5/(x)R/(y)H2Owherein n ranges from about 0.005 to about 0.6, m ranges from about 0.6 to about 1.2, x ranges from about 0.5 to about 0.99, and y ranges from about 10 to about 40. By using a molar ratio of the structure-directing-agent (R) over alumina less than 1, a surprisingly higher yield of silicoaluminophosphate molecular sieve can be achieved with less structure-directing-agent cost.